This invention relates to digital communication systems and more particularly to a digital radio system including apparatus for providing sync detection to allow synchronization and tracking of the transmitted digital signals.
Digital radio for carrier telephony appeared in the early 1970""s and was limited to modest spectral efficiencies and relatively short distances. The field has greatly grown over the past decades and the use of digital radio is widespread. In order to increase the efficiency of digital radio, digital engineers have raised the number of modulation levels and have generally dealt with modulation/demodulation techniques, spectral shaping and synchronization schemes. This has led to widespread and more efficient use of the digital radio systems.
As one can ascertain, a major problem in the transmission of digital data signals is interference due to noise. Hence, noise, in any communication system, plays an important part in the design and operation of the system. For general background on digital radio and particularly for microwave operation, reference is made to a text entitled Microwave Digital Radio edited by Srry J. Greenspan and Mansoor Shafi and published by the IEEE Press, the Institute of Electrical and Electronic Engineers, New York (1988).
During the past decade, many improvements have been implemented involving advanced digital radio techniques. Digital radio is used both commercially and for the military. As one can ascertain in dealing with digital data transmission, as data generated by computer systems and so on, one has to be extremely careful in processing the information so that the desired bits and the content of the bit patterns are easily recognized and decoded.
In order to do so, many systems operate with a synchronization system where a bit sync pattern is generated by the transmitting system and is detected by the receiving system. This bit sync pattern, once accessed at the receiver, synchronizes the receiver""s clocks so that all incoming digital data is sampled at the proper time and at the proper interval. This is extremely important in the transmission of digital data as compared to voice data, as voice data is just received, processed and listened to. As one can ascertain in regard to digital data, if the clock at the receiver drifts with respect to the transmitting clock then the data can not be properly decoded and processed.
Thus, a major specification of such systems is the reliable processing of digital data by performing the retrieval of a transmitted synchronization signal and then utilizing that signal to synchronize the clock or the timing generators at the local receiver. This is a common problem in communication systems of all kinds, and is particularly important in the formulation of digital data.
Nearly all microwave digital radio systems use linear modulation or those that are formed by translating base band pulse streams to IF or RF using balanced amplitude modulators. The principle of linear modulation is well-known and inputs I and Q represent sequences of data values in digital forms, with data values in each stream separated by T seconds. The systems use digital to analog and analog to digital converters which change the data streams into appropriate formats. The systems use a local oscillator which produces a sinusoidal carrier where it is applied to balanced modulators to provide the I and Q signals. Demodulators also utilize balanced circuits to demodulate the signal which is eventually applied to AD converters and which AD converters are sampled at a system sampling rate. Such digital signals are well-known and such digital systems are also well-known. In order to synchronize the transmit and receive data, phasing signals are employed to provide bit sync, and framing signals are employed to provide frame sync.
It is extremely desirable in all systems to synchronize the receiver clock with the transmitting clock so that sampling can be performed preferably at the center of the data bit to assure reliable decoding and to prevent noise from interfering with the retrieval of data at the receiver.
This is important in the transmission of digital data as compared to the transmission of voice. As indicated, such techniques are well-known including techniques for generating digital sequences and various methods for retrieving sync and framing data. This application relates to some novel approaches to generate and retrieve the sync data as well as frame sync by utilizing digital processing techniques as will be further explained.
It is therefore an object of this invention to provide an improved system operative with a digital radio or communication system which enables one to synchronize and maintain synchronization during a transmission between a transmitter and a receiver.
In a digital communications system having a transmitter for transmitting a digital data signal via a noisy transmission path, said signal having a phasing signal portion and a framing signal portion manifested by a repetitive bit pattern of ones and zeroes and used to synchronize a clock at a receiver responsive to said transmitted signal for receiving the same comprising means responsive to said phasing signal to provide an I and Q component and for correlating said components to provide an output signal indicative of the bit pattern in said phasing signal; tracking means responsive to said output signal and said receiving signal for periodically providing histograms of data bit transitions for producing an error signal indicative of difference of the transmitted clock rate and the sampling position of a received bit; and a clock means responsive to said error signal to provide a clock signal according to said error signal to enable sampling of received data bits relatively at the center of each bit.